1. Field of the Invention
This invention relates to a focus detecting device of the TTL (Through The Lens) type using a plurality of radiation sensitive elements in combination with a plurality of lens elements.
2. Description of the Prior Art
It is known in the art to provide a focus detecting device of the TTL type in the form of a radiation sensitive element arranged in a position optically equivalent to the position of the prescribed focal plane and operating to detect the degree of sharpness of an image formed on said radiation sensitive element, from which information representing the focusing condition of an image forming optical system is obtained. This form, because it largely depends upon the sensitivity of the radiation sensitive element, has many drawbacks.
On the other hand, as shown in U.S. Pat. No. 4,185,191 ("Range Detection System" by N. L. Stauffer, filed Jun. 5, 1978, patented Jan. 22, 1980) it is already known to provide a focus detecting device in which a number of lens elements are located adjacent to the prescribed focal plane of an image forming lens. For every one of these lens elements a respective pair of radiation sensitive elements arranged so that ones of the above-described radiation sensitive elements in each pair are made to receive light after having passed through one of two pupil openings symmetric to each other with respect to the optical axis in the above-described image forming lens at any time. The others of each pair of radiation sensitive elements are made to receive light after having passed through the other pupil opening at any time. The focusing condition of the above-described image forming lens to the object is detected by comparing the output of a group of those of the radiation sensitive elements which receive light from the one pupil opening with the output of another group of the other radiation sensitive elements which receive light from the other pupil opening, particularly on the basis of the phase relation of the both outputs.
This device of U.S. Pat. No. 4,185,191, unlike the before-mentioned image sharpness detection type, is of the type where the mutual positional relationship of the two images is detected. But, here, to realize such device, there are ever so many more requirements left still unfulfilled. For example, in such construction of the device as described above, for assurance of a sufficient performance and detection accuracy of the device, the plurality of lens elements and the plurality of radiation sensitive elements must be adjusted in position with high precision. Moreover, all the lens elements in the unit must be made up to precise tolerances on the order of .mu.m. Further a measure to meet the influence of variation of the ambient temperature must be taken into account. That is, in general, the array of the plurality of lens elements is manufactured from a transparent acrylic or other suitable synthetic resin plate with a great number of lenslets formed in one surface thereof, and, because of the fact that the acrylic or synthetic resin material is particularly susceptible to the influence of deformation due to the temperature variation, a counter-measure to it must be taken.